1. Field of the Invention
This invention relates, in general, to means, methods and materials for mounting electrical devices in packages; and, more particularly, to improved glass compositions, methods, and structures for bonding a semiconductor die to a ceramic base, and to improved semiconductor devices utilizing these materials, methods, and structure.
2. Description of the Prior Art
Metal, ceramics, glasses, and glass-ceramic mixtures are commonly used for packaging electrical devices such as semiconductor die in protective enclosures. The semiconductor die may comprise, for example, an individual element such as a diode, resistor, or transistor, an assembly of such elements, or may be an integrated circuit containing hundreds or thousands of elements. The package or protective enclosure can contain one or more semiconductor die and may have from two to a hundred or more external electrical leads.
The cerdip package is a common form of semiconductor device package widely used in industry today. It consists, typically, of an alumina ceramic base to which the semiconductor die is bonded, a lead frame for external contacts also bonded to the ceramic base, interconnections coupling the lead frame to the die, and a protective lid over the die and interconnections. Typical means used to bond the semiconductor die to the package base are: organic layers (e.g. metal or glass loaded epoxy;) glass layers (e.g. low temperature bonding glasses or glass-ceramic mixtures; or metal layers (e.g. metal layers evaporated on the semiconductor die and screened and fired on the ceramic base, and then alloyed together to fix the die to the base). Heat or heat and pressure are common means for forming the bond. Sometimes the bond is "scrubbed"; that is, the die is moved back and forth laterally in contact with the base, during bonding, in order to achieve a more homogenous bond region.
The physical characteristics of the bond region between the die and base are of great importance since they are a significant factor in determining the thermal impedance between the die where heat is generated and the exterior of the package base from which heat is extracted. Metal bonding layers, because they employ highly conductive materials, generally give lower thermal impedance. However, metal bonding layers use expensive materials and are more complex to make. Thus, devices utilizing them are more expensive. Bonding layers made of glass, are less costly but exhibit higher thermal impedance. Organic bonding layers exhibit still higher thermal impedance.
Measurements on a 64.times.64 mil (1.6.times.1.6 mm) silicon semiconductor die bonded to a 16-pin cerdip package base with a gold eutectic metal bonding layer gave a junction-to-case thermal impedance .theta..sub.JC of about 20.degree. C. per watt. The gold eutectic bond was about 2 mils (51 .mu.m thick). The .theta..sub.JC for the same chip and base bonded with a 2-3 mil (51-76 mm) thick glass layer of the prior art was 30.degree.-40.degree. C. per watt or higher. Type DIP-3, a commercially available bonding glass manufactured by Kyocera of Kyoto, Japan was used.
Attempts to lower the junction-to-case thermal impedance of prior art glass bonded die by making the glass layer thinner have been unsuccessful. The stress applied to the die due to the thermal expansion and contraction mismatch of the silicon and the alumina base depends on the glass thickness. The stress increases as the glass thickness decreases. Below about 2 mils (51 .mu.m), the stress exceeds the yield strength of the silicon semiconductor die and fracture occurs. Thus, with prior art glass bonding materials, thinner glass layers have not been practicable and improved .theta..sub.JC of semiconductor devices utilizing glass bonded die could not be achieved. Therefore, a need has continued to exist for means, methods, and materials for achieving improved glass bonding of semiconductor die and other components, and for achieving improved electrical devices having lower thermal impedance using glass bonding.
Accordingly, it is an object of the present invention to provide improved bonding and sealing material compositions for coupling electrical devices, particularly semiconductor die, to package bases.
It is a further object of the present invention to provide improved bonding and sealing material compositions for coupling electrical devices to an intermediate ductile foil which is in turn coupled to a ceramic package base.
It is an additional object of the present invention to provide improved bonding and sealing material compositions for coupling the ductile intermediate foil to the ceramic package base.
It is a further object of the present invention to provide an improved method for bonding a ductile foil to a ceramic base and, further, a method wherein the foil is substantially aluminum.
It is an additional object of the present invention to provide an improved method for attaching a semiconductor die to a ceramic base.
It is a further object of the present invention to provide an improved method for attaching a semiconductor die to a ceramic base using an intermediate ductile foil, sealed with or without use of an intermediate glass layer, the ductile foil being in turn sealed by another glass containing layer to the semiconductor die.
It is an additional object of the invention to provide an improved semiconductor device using a glass containing layer for die bonding which has a lower thermal impedance than the prior art.
It is a still further object of the present invention to provide an improved semiconductor device using a glass containing layer for die bonding with an intermediate ductile foil for simultaneously achieving stress relief and lower thermal impedance.
It is a still further object of the present invention to provide an improved semiconductor device using a glass containing layer for die bonding wherein the bonding layer can be thinner than has been possible in the prior art without die or bond cracking.